Another month another stone. Amethyst is the “traditional” birthstone for February and is the well known purple variety of the mineral quartz. Quartz, also known as rock crystal, is quite an interesting mineral, as it comes in many varieties, shapes and crystal habits.
From a geological point of view, this makes sense as quartz is made of the two most abundant elements in the Earth’s crust, oxygen at 46.6% and silicon at 27.7%, with a chemical formula SiO2. Anyone else see the 2:1 ratio here? If you want to really get to the scientific classification of minerals, this is how you could list amethyst.
Amethyst is identified as:
- Trigonal Crystal System (which is lumped into the Hexagon System do to the 3-fold symmetry). The Crystal System should never be confused with crystal habit. Habit is how the system will crystallize depending on the environment.
- Silicate Composition,
- Framework silicate Sub-class
- Quartz/Silica Family
- Mineral = Quartz – SiO2
- Variety = Amethyst.
Other known macro-crystalline varieties of quartz are; citrine, rose quartz, smoky quartz, rutilated quartz, milky quartz, prasiolite and ametrine. All of these varieties are classified are actually quartz but with different colour impurities.
Now, what do I mean by macrocrystalline? Well, quartz has the ability to create large, solid crystals and it can crystallize as tiny micro-crystals or spheres. These microcrystalline quartz varieties include agate, chalcedony, chrysoprase, carnelian, onyx, sardonyx, jasper, aventurine, bloodstone, tiger’s eye and moss agate. These varieties of quarts come in many colours, but that will be a blog for another month If you want to dig even deeper there are several sub-varieties of quartz characterized by temperature and pressure during crystallization.
But what makes amethyst purple, well that is due to the presence of iron, and some other trace element impurities within the crystal lattice/structure. Add a touch of radiation and voila, purple! Then if you want, add some heat of about 300-400 celsius and you will lose the purple and make citrine. SCIENCE! Interesting note, most citrine you see is actually heat-treated amethyst.
In the past, up until the 18th century, amethyst was classified as a cardinal gemstone, valuable gemstones that included diamond, sapphire, ruby, and emerald. This was due to its deep, regal purple hues. However, due to the discovery of extensive deposits in locations such as Brazil, amethyst lost most of its value. A perfect example of supply and demand when there is WAY too much supply. Amethyst price is not primarily defined by carat weight due to quantity. Instead, the colour is the biggest factor in determining the value of amethyst. The highest grade amethyst, called “Deep Russian” or “Siberian” is exceptionally rare but the value is still determined by the demand and is still much less than diamonds and sapphires.
This gemstone will always be precious and regal to me even if others disagree. Stay tuned for March when we talk about aquamarine, a variety of the mineral beryl.